tree: fix gui_synclist_scroll_stop called with uninitialized list

Regression introduced in 7a281ec.

Fixes crash with the "remember last folder" setting, when the
saved dir has since been removed:

exit_to_new_screen in dirbrowse calls gui_synclist_scroll_stop
when update_dir returns ≤0 for the number of dir entries.
But synclist has not been initialized by update_dir when it
returns early with -1 due to either tagtree_load or ft_load failing.

Move gui_synclist init further up in update_dir, so that when it
returns, the list is guaranteed to be initialized.

Change-Id: I62aa742a3d0121d5034440ff134992034e13fd90

apps/tree.c

@@ -413,6 +413,9 @@ static int update_dir(void)
     const bool id3db = false;
 #endif
 
+    /* Ensure that list is initialized before update_dir returns */
+    gui_synclist_init(list, &tree_get_filename, &tc, false, 1, NULL);
+
 #ifdef HAVE_TAGCACHE
     /* Checks for changes */
     if (id3db) {
@@ -461,8 +464,6 @@ static int update_dir(void)
         }
     }
 
-    gui_synclist_init(list, &tree_get_filename, &tc, false, 1, NULL);
-
 #ifdef HAVE_TAGCACHE
     if (id3db)
     {